Microcapsules are available usually in powder form and consist of spherical particles, which contain an encapsulated (entrapped) substance. The spherical particle usually consists of a polymeric shell and the encapsulated active product is located within the shell. The polymeric shell is frequently applied as a wall-forming material, and serves as a membrane for the encapsulated substance. This membrane is semi-permeable and allows the microcapsule to be an efficient tool for controlled release applications.
Microencapsulation itself has various advantages. Microcapsules protect sensitive substances from degradation processes and provide means for controlled release of desired active substances. It also enables the conversion of liquids to powders and is used to separate substances, which are harmful upon contact with each other.
Numerous techniques for microencapsulation are available depending on the nature of the encapsulated substance and on the type of polymer used. A widely used method for the encapsulation of water insoluble drugs within water insoluble polymers is the solvent removal method. Generally in such a process, the desired polymer is dissolved in a suitable organic solvent. This action is followed by the addition of the desired substance to be encapsulated. This substance is either dissolved or dispersed in the organic solvent. The resulting organic solution/dispersion is dispersed in an aqueous phase to obtain an-oil-in-water emulsion where oily microparticles are dispersed in the aqueous phase. Upon complete removal of the solvent from the microparticles the microcapsules are formed. Several patents describe methods of removing the solvent. U.S. Pat. No. 4,384,975 describes the removal of the solvent by vacuum distillation. In GB 1,394,780 the removal of the solvent is done by evaporation. In U.S. Pat. No. 3,891,570 the removal of the polymer solvent is carried out by heating the aqueous dispersion or by reducing its pressure. In U.S. Pat. No. 3,737,337 the removal of the organic solvent is done by extraction with water, however-it is limited to certain solvent systems. U.S. Pat. No. 5,916,598 describes a method for the preparation of slow-release biodegradable, biocompatible microparticles using the general technique of solvent removing, and microparticles comprising microencapsulated active agent.
Microencapsulation is suitable for drugs, vitamins and food supplements since this process is easily adaptable by varying the solvents and/or the polymers. It may yield microcapsules having desirable size, round shape and smooth surface that are important for controlled release and for the chemical stability of the core material.
A basic prerequisite for this process is the use of a solvent that is able to dissolve efficiently the substance to be encapsulated as well as the wall-forming material. This solvent has to be only partially soluble in water, giving rise to emulgation of an organic phase in a continuous water phase. Chlorinated solvents such as dichloromethane, chloroform, or their mixtures with other solvents are widely used since they facilitate the microencapsulation process.
However all the microencapsulating technologies based on solvent systems such as chlorinated solvents are not applicable for food, cosmetic and pharmaceutical applications, since they do not meet FDA and other regulations due to the presence of residual amounts of chlorinated solvents in the microcapsules. Simple vacuum or heat drying do not result in a sufficiently low chlorinated solvent content so as to meet FDA regulations, thus creating an essential necessity for a method for encapsulating vitamins, food supplements, oils or pharmaceuticals by the solvent removal technique.